[Cerowrt-devel] [Bloat] [Codel] FQ_Codel lwn draft article review

[Alex Burr]

[Oops, intended to CC the list.]

An extra 15-20ms of latency, which is what you're seeing there, 
shouldn't be caused by packet overhead in the actual VDSL modem part of 
the device. Worst case the implementation of the PTM-TC layer (the 
equivalent of ATM) might round the packet up to a multiple of 64 bytes 
(although it's not supposed to), but unless your line rate is 25kbps, 
that should not cause 20ms of latency. 

VDSL2 operates at 4Khz or
 8Khz symbol rate, and while it contains and interleaving or 
retransmission layer which can add up to 64ms in its most high-latency 
configuration, those layers do not operate at the level of packets and 
I'm pretty sure that that latency cannot be affected by packet size in a
 conformant implementation.
This latency is being caused by something else. 

The
 best way to figure out the per-packet overhead in your VDSL2 modem is 
probably  to count the number of packets that get through, not measure 
latency. 

Alex



----- Original Message -----
> From: Dan Siemon <dan at coverfire.com>
> To: bloat <bloat at lists.bufferbloat.net>; codel at lists.bufferbloat.net; cerowrt-devel at lists.bufferbloat.net
> Cc: 
> Sent: Tuesday, December 4, 2012 3:13 AM
> Subject: Re: [Bloat] [Codel] [Cerowrt-devel] FQ_Codel lwn draft article review
> 
> On Mon, 2012-12-03 at 15:58 +0000, David Woodhouse wrote:
>>  ADSL is basically just ATM with a strange PHY. You have a bunch of
>>  options for how you use this ATM link. Mostly it's RFC2364 PPP-over-ATM
>>  or it's PPPoE on top of RFC2684 Ethernet-over-ATM.
> 
> Speaking of xDSL, does anyone on the list happen to have a good
> understanding of how much per-packet overhead there is on VDSL2? I've
> been tweaking the buffering and shaping on my upstream link and noticed
> unexpected behavior with small packets.
> 
> The link below (use wayback machine version) has a good description of
> per-packet overhead for various forms of ADSL but I haven't found
> something similar for more modern DSL variants.
> http://www.adsl-optimizer.dk/thesis/
> http://web.archive.org/web/20090422131547/http://www.adsl-optimizer.dk/thesis/
> 
> I started a discussion on DSLReports
> http://www.dslreports.com/forum/r27565251-Internet-Per-packet-overhead-on-Bell-s-VDSL-ATM-based-
> but experimentally the overhead discussed there doesn't appear to be
> correct
> http://www.coverfire.com/archives/2012/11/29/per-packet-overhead-on-vdsl2/
> 
> 
> 
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